[time-nuts] HP 5061A Problem

Adrian rfnuts at arcor.de
Wed Oct 9 05:38:42 EDT 2013


the frequency test performed yesterday as follows:

- Peak Frequency 38.78 kHz at 1V eff
- EM Output connected directly to a 34401A
- measured -102 mV (10.2 nA)

- Peak Frequency 38.78 kHz at 2V eff
- EM Output connected directly to a 34401A
- measured -133 mV (13.3 nA)

- Audio Signal off
- EM Output connected directly to a 34401A
- measured -59 mV (5.9 nA)

Corresponding circuit check meter readings when EM output is connected 
to A7 J1:
- 36
- 46
- 24

That was measured with A11 mass spectrometer fully cw. There is no peak 
when adjusting the pot, the max. is always in the full cw position.

There was still no 2nd harmonic though because the A7 input amp was shot.
I couldn't find the 1854-0023 in the xref, so I replaced them with BC546B's.
This worked until I reconnected the EM output to the amp.
Apparently the EM charges up to a high negative potential while 
unconnected. When it is reconnected to A7, the negative voltage peak 
instantly shoots the input transistor.
Touching the open connector with a scope probe shows a negative voltage 
transient of several hundred volts.
When it is connected to the amp, there are only -8mV across the 1M input 
resistor R10.
So there appears to be a very high resistance path between the EM and 
high voltage inside the tube.
This explains also why the meter is displaying beam current immediately 
after applying power to the 5061A.

Well, with a new transistor and avoiding the discharge scenario, I was 
finally able to get a 2nd harmonic that could be peaked with osc. 
coarse, A3 mod and A4.
The noisy 2nd harmonic signal is present at A8 J1.

So far it looks all good except for the EM problem that appears to be 
acceptable. At least the hope is still alive.

The unit didn't yet lock, so I will have to check the other parts of the 
control loop.


cdelect at juno.com schrieb:
> Adrian,
> Most used High stability tubes are DOA but you might get lucky!
> You say the low frequency test "worked".
> What was the amplitude of the peak in nanoamps and was the peak well
> above the noise or baseline level?
> Once you were on the peak did you adjust the 41.6Khz amplitude to max the
> signal?
> Once on the peak you can adjust the mass spec pot on the top of A11 to
> peak it.
> If the peak was >8na and above noise then the tube appears to be working.
> Anything below 4 or 5 na the tube is depleted, might be useable but STS
> will be terrible.
> If the peak is >8na then reconfigure for normal operation, in open loop
> mode.
> Turn the mod on and connect a scope to A8 J1 to observe the phase
> detector output.
> Adjust the fine and or coarse oscillator adjustments back and forth
> across 5Mhz.
> If you can see any output as you sweep through then set the osc.
> frequency to peak the signal.
> Then adjust the A3 mod and the A4 (top pot) to peak the signal.
> A7 gain will adjust the amplitude.
> Adj the osc freq. to null the signal (puts you "on" frequency) and see if
> you have any 2nd harmonic now.
> If so go ahead and see if it will lock.
> Good Luck!
> Corby
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